How does std::bind handle member functions, and why is an object reference required?

How std::bind Works with Member Functions

When using std::bind with member class functions, several key concepts come into play:

The First Argument: A Pointer

In the syntax std::bind(&Foo::print_sum, &foo, 95, _1), the first argument is not a reference to a function pointer, as one might assume. Instead, it's a pointer representing the member function itself.

The Second Argument: An Object Reference

The second argument, in this case &foo, is a reference to an object of the appropriate class (Foo). This is necessary because a member function requires an object instance to operate on.

How std::bind Handles Member Functions

Internally, std::bind detects that the first argument is a pointer to a member function and converts it into a callable object using std::mem_fn. This callable object retains the pointer to the member function and requires an object instance as its first argument.

The Need for an Object Reference

When binding to a member function, an object reference becomes essential because the member function is associated with a specific class and requires an object of that class to be invoked. The second argument to std::bind provides this object reference, allowing the member function to operate on the specified object.

Example

Consider the following code:

<code class="cpp">struct Foo {
    int value;
    void f(int n) { std::cout << "f(" << this->value << ", " << n << ")\n"; }
};

int main() {
    Foo foo{10};

    auto f_bound = std::bind(&Foo::f, &foo, _1);
    f_bound(20); // Output: f(10, 20)
}</code>

In this example, f_bound is a callable object that invokes the f member function on the foo object with any additional arguments it receives. So, when we call f_bound(20), it's equivalent to calling foo.f(20), which prints "f(10, 20)".

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